1. Field of Invention
Exemplary embodiments of the invention relate generally to a system and method for determining a position of a mobile unit. For example, embodiments relate to techniques for estimating a position of a railway vehicle in relation to a point of interest.
2. Discussion of Art
Conventional positioning of a moving vehicle may be based, for example, on speed and position measurements, Doppler effect, the Global Positioning System (GPS), or some combination of these. One common approach to estimating a vehicle position uses the Global Positioning System (GPS). However, GPS requires a line of sight between the receiver and the global navigation satellites. Furthermore, the accuracy provided by GPS for civilian use is limited. Another common system for determining vehicle position, referred to an inertial navigation system (INS), makes use of a gyroscope and accelerometer. One advantage of an INS is that there is no need for line of sight to an external device. However, inertial systems have unbounded position error that increases slowly with time, which means decreasing accuracy.
Other common systems for determining vehicle position include the use of speed and position measurements taken using a tachometer. Position estimates can also be achieved by transmitting measurement signals using transponders or track circuits. However, such techniques present various obstacles to obtaining accurate position estimates for a high-speed vehicle. For example, track circuit methods can only achieve accuracy equal to a length of a track block. In tachometer-based systems, position errors occur caused by slipping of the vehicle wheals. Position measurement errors may be corrected by using GPS signal. However, in many applications, including railway applications, line of sight may not always be available.